A technique that is now widely used for insulating electrical conductors such as wires, and for producing coatings for other purposes and on other substrates, entails the exposure of a grounded workpiece to a cloud of electrostatically charged fusible particles, thereby causing the particles to deposit thereupon for subsequent integration. Typical of the apparatus used for that purpose are the devices disclosed and claimed in Knudsen and Karr U.S. Pat. Nos. 3,916,826 and 4,030,446, respectively; electrostatic fluidized bed equipment and systems that are highly effective for such coating are commercially available from Electrostatic Technology Incorporated, of New Haven, Conn.
A well-recognized problem associated with the electrostatic fluidized bed technique concerns the achievement of a uniform build upon the workpiece. The problem is most significant from the standpoint of achieving top-to-bottom uniformity, the lower surfaces tending to develop a heavier build than the upper surfaces, essentially because they are closest to the source of the particle cloud. This is believed to be attributable to two effects, one being the rarefaction or decrease in density of the cloud upwardly over the bed, and the other being a decreasing value of average electrostatic charge as the particles rise in the bed, due either to increasing remoteness from the voltage source or to dissipation of the original charge, or both.
The prior art has recognized these characteristics of electrostatic fluidized bed coating, and has proposed various solutions. Effective approaches are described in U.S. Pat. Nos. 4,297,386 and 4,330,567, to Gillette, No. 4,332,835 to Knudsen, and Nos. 4,418,642 and 4,472,452 to Gillette et al, wherein the nature of the particle cloud is controlled by electrical means. In U.S. Pat. No. 4,084,019, Christ et al employ electrode grids buried within the powder bed to form rows of localized corona discharges to either side of a passing substrate.
It is also common practice to mask the workpiece to control build, by interposing a physical barrier between it and the cloud. This as may be done by passing a wire to be coated through a tubular member, the extension of which into the coating chamber may be altered to vary the effective length of the workpiece exposed; such a method is described, for example, in Beebe et al U.S. Pat. Nos. 3,396,699, 3,566,833, and Voelker et al U.S. Pat. No. 4,329,377. Although the tubes utilized therein create a condition of either full exposure or full masking of the enclosed length of the workpiece, means for masking only a portion of the periphery is also known, as is disclosed in U.S. Pat. Nos. 3,828,729 to Goodridge, 4,011,832, to Westervelt, et al and 4,051,809 to Zicar et al, which also show baffles oriented to deflect the upwardly moving stream of particles over the top of the workpiece being coated. Hajek discloses an improved apparatus and method in U.S. application Ser. No. 6/543,858 (now U.S. Pat. No. 4,517,219) wherein a peripherally configured rectilinear bar is used for build control. In any event, the configuration of the build control means utilized, as well as the effective distance over which it influences the deposit on the workpiece, will have a very significant effect upon the nature of the coating produced.
The prior art discloses techniques, in addition to the foregoing, which also have the objective of producing uniform coatings upon articles of various kinds. For example, in U.S. Pat. No. 2,777,784, Miller teaches a method and apparatus in which an elongated article is surrounded by an atomizing edge, which may be in the form of a continuous helix encircling the travel path, to produce a coating by electrostatic attraction. In Barford et al U.S. Pat. No. 3,248,253, a workpiece, which may be wire, is conveyed through an annular arrangement of charging electrodes immersed within a powder bath (see FIGS. 5 and 6).
Guns and nozzles are of course also used for electrostatic coating, and it has been proposed to employ a number of them at spaced positions about the workpiece, as in U.S. Pat. Nos. 2,421,787 to Helmuth, 3,155,545 to Rocks et al, 3,439,649 to Probst et al, and 3,607,998 to Goodridge. Inoue describes an electrostatic spray device in U.S. Pat. No. 3,326,182, including a housing for directing a gas stream toward a surface to be sprayed; radially inclined apertures are used to introduce ionized particles into a discharge chamber of the housing, so that the axially propagated spray from a coaxial nozzle is displaced spiroidally in a vortex (column 3, lines 30-56).
Putney teaches a fluidized bed coating method, in U.S. Pat. No. 3,834,927, wherein the aerating gas is constrained to enter the bottom of the bed at a localized influx zone to promote uniformity in the bed, and hence in the deposit produced. Finally, in U.S. Pat. No. 4,034,703 Schieber et al disclose apparatus for coating elongated metal members utilizing a head immersed in the bed of powder, which has annular nozzles through which the particles are induced to flow onto the surface of the article.
Although at least certain of the foregoing methods and apparatus offer, to a greater or lesser extent, decided advantages over earlier practices, still the consistent attainment of coatings that conform to close thickness tolerances, and that are effectively isolated from external influences, remains a goal that has not been fully achieved. Thus, despite all of the activity evidenced by the foregoing a need remains for a method and apparatus for producing coatings of highly uniform thickness by electrostatic powder deposition, the quality of which is not unduly affected by changes in the position of the workpiece within the cloud of charged particles (particularly vertical spacing above a fluidized bed), from aberrant voltage and frequency variations experienced by the electrical system, and the like.
Accordingly, it is a primary object of the present invention to provide a novel method, apparatus, and system by which workpieces, and particularly conductors of continuous length, can be coated by electrostatic powder deposition, quickly, efficiently, safely, and with an exceptionally high degree of uniformity in the build.
It is also an object of the invention to provide such a novel method, apparatus and system, wherein the nature of the coating can readily be controlled by the speed of the workpiece and the magnitude of the voltage applied, is highly tolerant of changes of workpiece position within the cloud of charged particles, and is virtually unaffected by normal fugitive electrical effects, such as noise and static.
Another object of the invention is to provide such a method, apparatus and system wherein coating can be carried out in an electrostatic fluidized bed, at voltage levels that are significantly reduced from those heretofore employed for practical high-speed operation, thereby enhancing safety.
Still another object of the invention is to provide such a method, apparatus and system wherein economy of production is maximized by the significant reduction of waste produced during start-up and discontinuances of operation.
A still further object is to provide a novel coating unit which is uncomplicated and relatively inexpensive to manufacture and operate.